A Rocky Planet Around One of Our Galaxy’s Oldest Stars 

University of Hawaiʻi at Mānoa
Roy Gal , (808) 388-8690
Associate Astronomer , Institute for Astronomy
Posted: Jan 11, 2021

Photo 1, caption and credit in text*
Photo 1, caption and credit in text*
Photo 2, caption and credit in text*
Photo 2, caption and credit in text*

University of Hawaiʻi Astronomers Using W. M. Keck Observatory Discover Ancient Magma World Orbiting a Chemically Unusual Star

Maunakea, Hawaii - “They should have sent a poet,” says Ellie Arroway in the film Contact as, suspended in outer space, she gazes upon a spiral galaxy.  Almost all of the planets discovered to date (including the solar system planets) are confined to the plane of the Milky Way, unable to glimpse such a sweeping vista of our galaxy.  However, astronomers at the University of Hawaiʻi Institute for Astronomy (IfA) using the W. M. Keck Observatory on Maunakea have discovered a rocky planet with a different kind of view.

The planet orbits the star TESS Object of Interest (TOI) 561, named for the ongoing NASA TESS planet hunting mission.  TOI-561 belongs to a rare population of stars called the galactic thick disk.  Thick disk stars are chemically distinct, with fewer trace heavy elements (and especially less iron) than typical stars of the Milky Way, suggesting they formed early, approximately 10 billion years ago.  They also have wandering motions that can lift them out of the galactic plane, providing an epic view of our own spiral galaxy.

“The rocky planet orbiting TOI-561 is one of the oldest rocky planets yet discovered.  Its existence shows that the universe has been forming rocky planets almost since its inception 14 billion years ago,” says Dr. Lauren Weiss, Beatrice Watson Parrent Postdoctoral Fellow at UH IfA and leader of the team that discovered the TOI-561 planetary system.  

The result was announced at a press conference today at the January 2021 meeting of the American Astronomical Society and is published in the Astronomical Journal.

The rocky planet orbiting TOI-561 transits its star, meaning that the planet passes in front of its star as seen from Earth, blocking a fraction of the starlight.  The planet is small, with a radius only one and a half times that of Earth. As a result, the reduction of light it causes is miniscule, just 0.025% of the star’s brightness.  

Astronomers at UH IfA noticed this change in intensity and used Keck Observatory’s High Resolution Echelle Spectrometer (HIRES) to confirm the presence of the planet. By measuring the wobble of the star induced by the planet’s gravity, they were able to infer that the planet has three times the mass of Earth.  Combining this mass with the radius determined from the transits, the team concluded that the planet is most likely rocky, perhaps with less iron than Earth.

TOI-561 has at least two other planets transiting the star, both of which have about twice Earth’s radius and are too large and low-mass to be rocky.

The origin of stars in the galactic thick disk is unclear, with some studies suggesting that they formed in a distinct, old galaxy that our younger and more massive Milky Way galaxy later cannibalized.  Alternatively, they could be some of the first stars that formed within the Milky Way.  “I wonder what view of the night sky would have been accessible from the rocky planet during its history,” adds Weiss.

Although the past 10 billion years of the planet’s history are murky, it likely does not host life now. The planet orbits its star twice every Earth day, so close to its host star that the estimated average surface temperature is over 2000 degrees Kelvin--far too toasty for life as we know it.  However, this rocky, magma world is perhaps a harbinger of a population of rocky worlds yet to be discovered around our galaxy’s oldest stars.

The observations and analysis in this study were carried out as part of the TESS-Keck survey, a multi-institutional effort to characterize the properties of planets discovered by the NASA TESS mission.



Founded in 1967, the Institute for Astronomy at the University of Hawaiʻi at Mānoa conducts research into galaxies, cosmology, stars, planets, and the sun. Its faculty and staff are also involved in astronomy education, deep space missions, and in the development and management of the observatories on Haleakalā and Maunakea. The Institute operates facilities on the islands of Oʻahu, Maui, and Hawaiʻi.


W. M. Keck Observatory’s High-Resolution Echelle Spectrometer (HIRES) produces spectra of single objects at very high spectral resolution yet covers a wide wavelength range. It does this by separating the light into many "stripes" of spectra stacked across a mosaic of three large CCD detectors. HIRES is famous for finding exoplanets. Astronomers also use HIRES to study important astrophysical phenomena like distant galaxies and quasars, as well as find cosmological clues about the structure of the early universe, just after the Big Bang.


The W. M. Keck Observatory telescopes are among the most scientifically productive on Earth. The two 10-meter optical/infrared telescopes atop Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. For more information, visit:



Caption: Artist’s rendition of TOI-561, one of the oldest, most metal-poor planetary systems discovered yet in the Milky Way galaxy. This 10 billion-year-old system has a hot, rocky exoplanet (center) that’s one and a half times the size of Earth as well as two gas planets (to the left of the rocky planet) that are about twice as large as Earth. Credit: W. M. Keck Observatory/Adam Makarenko


Caption: Illustration showing the structural components of the Milky Way Galaxy. The star TOI-561 is located in the thick disk (marked in red-orange), which contains a rare, older population of stars. While nearly all known planets are found within the thin disk (marked in orange), the newly-discovered rock-and-lava exoplanet orbiting TOI-561 is one of the first confirmed rocky planets orbiting a galactic thick disk star. Credit: Kaley Brauer, MIT